Aqueous inks are often the most suitable type for the process of desktop printing, which has become nearly ubiquitous in home and office environments with the advanced personal computers and inkjet printers now available. Possessing a hydrophilic nature in order to absorb the ink, ink-receptive media are in dynamic equilibrium with the environment with regard to moisture. Hydrophilic materials tend to expand as moisture is absorbed. Differential expansion within a multi-layered material will result in curl. Usually photo-quality media are intended for printing on one side only and hence the expensive coatings necessary to provide excellent image quality are provided on only one side of the recording material. Differential absorption of environmental moisture by asymmetrically-coated media may lead to undesirable curl. Even symmetrically-coated media may curl when printed with aqueous ink on one side.
For aesthetic as well as practical reasons, curl of inkjet photo paper is undesirable. Coated paper may exhibit a tendency to curl in response to environmental factors, most importantly the relative humidity, or in response to wetting or drying, for example, during printing or during manufacture. Curl may be induced in a coating through unsymmetrical treatment during the manufacturing stage or by humidity changes prior to its use in a printer. Curled paper may be unsuitable for accurate picking and feeding by the printer. Even uncurled paper may curl upon rewetting by applied ink and produce a head strike or fail to transport. Finally, curled prints may collide in the output tray of the printer. For these reasons, the phenomenon of curl can be a complex phenomenon. Nevertheless, manufacturers wish to minimize the curl of inkjet receivers under a variety of conditions.
A back coating on the side of the support opposite to the image-receiving layer is known to provide a reduction in curl by balancing the curl propensity of both sides of the recording sheet. For example, Idei et al. disclose, in U.S. Pat. No. 5,302,437, a back coat layer comprising inorganic particles and a binder and recommend that the difference in coating weight between front and back coatings be minimized. Even though the curl may be reduced, problems with reliable picking and feeding of media by a printer transport mechanism can remain.
Pre-cut standard-size recording materials held in a stack are universally accepted for desktop printers as a matter of convenience. The printer transport mechanism includes means for picking one sheet from the supply stack at the beginning of the printing process. Reliable picking of exactly one sheet requires that the sheet maintains non-slip contact with the printer transport mechanism, usually including a set of compliant rollers, and easily slides from the media supply stack. Failure to “pick” or the picking of multiple sheets at one time are problems arising from recording materials with surface properties unsuitable for manipulation by the printer's sheet-feeding mechanism.
The sliding of one sheet from a stack is influenced by the front-to-back static coefficient of friction and the adhesion of the backside to the transport roller. Owatari et al. disclose, in U.S. Pat. No. 5,928,787, a backside coating of aqueous binder and higher fatty acid salt. The problem with this approach is that large amounts of expensive polymer are needed to compensate for the curl propensity of high coat weights on the front side of photographic quality media.
Ishiyama et al. disclose, in U.S. Pat. No. 6,436,514, a porous image-recording element with an ink-receiving layer and a gloss layer on the front side of the support and on the side of the support opposite these coated layers, a layer comprising a pigment and a binder, where the static coefficient of friction between the gloss layer and the back-coat layer is 0.9 or less under the conditions of 20° C. and 65% relative humidity (“RH”).
When the binder amount in the backside layer is low, transport in the printer can tend to be unreliable under conditions of low temperature and low humidity. Furthermore, the integrity of the backside layer may be low, resulting in the problem of dust formation in the manufacturing operations of slitting and cutting the media and in transporting the media through a printer. Another problem with low backside integrity is the flaking of the backside when an adhesive is applied to the backside for mounting purposes. On the other hand, if the binder level in the coating composition for the back side is significantly increased, the viscosity of the coating composition increases, and its composition must be diluted, resulting in an increased drying requirement or reduced manufacturing productivity.